This invention relates generally to methods of and apparatus for controlling the gap between the mandrel and die during extrusion, and in particular relates to methods of and apparatus for controlling the gap between a mandrel and die during the continuous extrusion of tubing from a larger to smaller diameter.
As known to those skilled in the extrusion art, especially those skilled in the art of continuously extruding large diameter tubing into smaller diameter tubing, for the extrusion of uniform product at a minimum pressure with good speed control, the gap between the mandrel and die must remain constant. As is further known to those skilled in the extrusion art, the extrusion process sets up dynamic forces which act on the mandrel and die causing the actual gap therebetween to be larger and smaller than the desired constant gap at different times thereby causing large variation in extrusion pressure.
More particularly, and as is known to those skilled in the art, upon the commencement of the extrusion process with the extrusion pressure going from zero to maximum, the mandrel and die compress thereby causing the actual gap therebetween to be larger than the desired gap whereby the material being extruded therebetween work hardens producing redundant work which makes the material less deformable and causes the extrusion pressure to rise producing extrusion control problems; extrusion conditions can arise causing the actual gap between the mandrel and die to be smaller than the desired gap, such as for example upon the commencement of extrusion with the product initially flowing over the mandrel, friction drag forces are produced on the mandrel causing it to move toward the die making the actual gap therebetween smaller than the desired gap and effectively creating a "check valve" between the mandrel and die causing the extrusion pressure to rise exponentially thereby again causing extrusion control problems.
Gap control apparatus and methods are known to the prior art, for example those disclosed in U.S. Pat. No. 3,950,979 issued Apr. 20, 1976 to F. J. Fuchs, Jr.; however, the invention disclosed therein is for controlling the die gap upon the actual gap being larger than the desired gap and includes no teaching of how to control the actual gap upon the actual gap becoming smaller than the desired gap.
Accordingly, there exists a need in such extrusion art for methods of and apparatus for controlling the actual gap between the mandrel and die during extrusion and for re-establishing the desired gap upon the actual gap becoming larger or smaller than the desired gap.
As is still further known to those skilled in the art, in the typical extrusion operation particularly a continuous extrusion operation, the die is typically of relatively short length but is supported by a die stem of considerably larger length and it is the die stem which experiences the above-noted compression causing the gap control problems. Still further, as is also known to those skilled in the art, the mandrel is typically of considerable length and the above-noted compression occurs over the length of the entire mandrel. Accordingly, it has been found to be highly desirable, if not required, that the apparatus for sensing the actual gap between the mandrel and die be positioned as close as reasonably possible to the actual gap or the area sometimes referred to in the art as the zone of deformation.
Accordingly, it is a further object of the present invention to provide methods of and apparatus for controlling the gap between a mandrel and die whereby the sensing of the actual gap is performed in close proximity to the actual gap or the zone of deformation.